The present invention generally relates to a magnetic lens and, in particular, relates to a magnetic lens having a preselected thermal power dissipation independent of the strength of its magnetic field.
Magnetic lenses are generally known and are often employed to focus a beam of charged particles i.e. electrons or ions. One major use of a magnetic lens is in analytical instruments which utilize focused electron beams to stimulate a measurable reaction from a sample of material which is being elementally analyzed. In such instruments the magnetic lens is useful as an objective, or final focusing control. That is, the objective magnetic lens is the last beam diameter control element prior to the beam impinging on the sample. As such, the stability of the beam position as affected by the magnetic lens is quite critical to achieve an accurate characterization of small features on the sample.
One feature of conventional analytical instruments employing an electron beam is that the energy of the electron beam is variable over a range of energies consistent with the range of materials to be characterized. For example, in an Auger analysis, the energy of the electron beam determines the depth of penetration and the range of Auger excitation of the atoms of the sample. Since the energy necessary to produce Auger electrons varies from atom to atom, a beam generator, to be effective, must be capable of producing electrons of various energies. In addition, the beam energy must be variable if the sample under analysis is an insulator, if the beam energy is sufficiently high, can become electrostatically charged. Such charging can be avoided by reducing the beam energy, sometimes to as low as 2 KeV. Another instance which mandates a variable beam energy is where an X-ray detector is used. In order to produce atomic emissions in the X-ray spectrum, the energy of the charged particle beam must be quite high, for example, on the order of about 30 KeV. Thus, to be practical, the energy of, for example, an electron beam should be variable over a broad energy range.
As one might expect, however, when the beam energy or beam size is changed, the strength of the magnetic field of the magnetic lens must also be adjusted in order to ensure that the beam having the changed energy remains focused on the same point of the sample as the beam of previous energy. In conventional magnetic lenses, the strength of the magnetic field is controlled by the current passing through a single coil of wire defining an electromagnet.
It is well known that one major consideration in the design of a magnetic lens is the removal and dissipation of the thermal power generated thereby. If the power dissipation mechanism is not sufficient, the position of the beam drifts until equilibrium is reached. In most instances, the reaction time of the thermal dissipation mechanism is inadequate to readily keep pace with the thermal changes in the magnetic lens caused by changing the strength of the magnetic field in response to a change in beam energy.
One known mechanism used to stabilize the temperature of a magnetic lens is to introduce a water cooling jacket around the lens. Such a mechanism, while substantially effective, is nevertheless cumbersome, not only in its initial design but also in its implementation. In addition, cooling systems in general often require frequent maintenance and repair due, in part, to impurities and corrosive material in the working fluid. Hence, the reliability of such cooling means is questionable thereby reducing the reliability of the entire instrument.
Hence, conventional magnetic lenses can require an extraordinary amount of time to stabilize whenever the beam energy is changed. This problem causes a considerable loss of time during the analysis of a single sample.